SQL> WITH test
  2       AS (SELECT 1 empno, 'ramesh' empname, 5000 salary FROM DUAL
  3           UNION ALL
  4           SELECT 2, 'hari', 3000 FROM DUAL
  5           UNION ALL
  6           SELECT 3, 'kiran', 6000 FROM DUAL
  7           UNION ALL
  8           SELECT 4, 'vamsi', 2000 FROM DUAL),
  9       ranked
 10       AS (SELECT empno,
 11                  empname,
 12                  salary,
 13                  RANK () OVER (ORDER BY salary) rnk
 14             FROM test)
 15  SELECT SUM (salary)
 16    FROM ranked
 17   WHERE rnk < 3;

SUM(SALARY)
-----------
       5000

SQL>